redis源码阅读-sds(简单动态字符串)

¶redis源码阅读-sds(简单动态字符串)

¶介绍

sds是redis定义的代替c语言标准字符串的简单动态字符串
在redis中，c语言标准字符串通常被作为字面量，而字符串的修改则使用sds来进行
sds字符串有简单的数据结构，可以看作是一个封装类

¶sds.h

/* SDSLib 2.0 -- A C dynamic strings library
 *
 * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
 * Copyright (c) 2015, Oran Agra
 * Copyright (c) 2015, Redis Labs, Inc
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */
/**************************************************
 * redis自己构建的简单动态字符串抽象类型，用来代替c语言传统字符串
 * 在redis中，c字符串只作为字面量，sds用来被表示可被修改的字符串
 **************************************************/
#ifndef __SDS_H
#define __SDS_H

// 为sds预留的最大分配内存
#define SDS_MAX_PREALLOC (1024*1024)
const char *SDS_NOINIT;

#include <sys/types.h>
#include <stdarg.h>
#include <stdint.h>

// 将char* 指针自定义为sds类型
typedef char *sds;

// __attribute__ ((__packed__))关键字 https://blog.csdn.net/weixin_39533180/article/details/76207099
// 功能： 结构体建立的时候，会进行字节对齐。为了减少内存占用，使用这个关键字取消字节对齐，按照紧凑排列的方式，占用内存

/* Note: sdshdr5 is never used, we just access the flags byte directly.
 * However is here to document the layout of type 5 SDS strings. */
/**
 * lsb(Linux Standards Base/ least significant bit)和msb(Most Significant Bit)
 * 参考：https://zhidao.baidu.com/question/155072477.html
 * lsb: 最低有效位,msb: 最高有效位,MSB位于二进制数的最左侧，LSB位于二进制数的最右侧。
 *
 * sdshdr使用场景分析 : https://www.codercto.com/a/46648.html
 */

/* sds的五种类型
 * len : sds的长度，不包括结尾结束符
 * alloc : 分配的sds长度,不包括结束符
 * flag: sds类型
 * buf: sds实际存放位置
 */
// flag低三位保存sds类型，flag高三位保存string长度(buf)
struct __attribute__ ((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char buf[];
};
// uint8_t: 无符号char (-128~127) char默认是 signed char
struct __attribute__ ((__packed__)) sdshdr8 {
    uint8_t len; /* used */
    uint8_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint16_t : short int (占2字节)
struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint32_t : int (占4字节)
struct __attribute__ ((__packed__)) sdshdr32 {
    uint32_t len; /* used */
    uint32_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint64_t : long int (占8字节)
struct __attribute__ ((__packed__)) sdshdr64 {
    uint64_t len; /* used */
    uint64_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};

// 为sds类型定义标志码
#define SDS_TYPE_5  0
#define SDS_TYPE_8  1
#define SDS_TYPE_16 2
#define SDS_TYPE_32 3
#define SDS_TYPE_64 4
#define SDS_TYPE_MASK 7
#define SDS_TYPE_BITS 3

/**
 * 这三个函数共通的地方就是功能都是为了去除sds类型和结构，获得有效的数据
 *
 * SDS_HDR_VAR(T,s)/SDS_HDR(T,s)
 * T传入的是(5,8,16,32,64),sdshdr##T拼接为sdshdr5,sdshdr8,sdshdr16,sdshdr32,sdshdr64
 * s传入的是字符串
 * (void*)((s)-(sizeof(struct sdshdr##T))) 是为了将字符串的sds结构体所占大小减去，获得字符串实际长度
 *
 * SDS_HDR(T,s)和SDS_HDR_VAR(T,s)区别就是SDS_HDR_VAR(T,s)是需要变量中转，SDS_HDR(T,s)不需要变量中转
 */
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
// f 是传入的flag,(f)>>3表示将3位最低有效位(存储sds类型)消除,获得没有类型的长度
#define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)

// 获取sds长度
static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    // flag存储的是sds五种类型的二进制 5(0000),8(0001),16(0010),32(0011),64(0100)
    // flag&SDS_TYPE_MASK = 0000 & 0111 = 0000 ; 最终得到的值就是flag。SDS_TYPE_MASK是屏蔽位，不需要检查的位置0，检查到对应的位对应置1
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}

// 获取sds可用字节数(分配长度-已经分配的长度)
static inline size_t sdsavail(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: {
            return 0;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            return sh->alloc - sh->len;
        }
    }
    return 0;
}

// 为sds设置长度
static inline void sdssetlen(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: // sdshdr5没有len属性，不能直接给其赋值,需要按相反的操作来
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS); // 加上最低有效位(sds类型)
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len = newlen;
            break;
    }
}

// sds在原有基础上增加inc的长度
static inline void sdsinclen(sds s, size_t inc) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len += inc;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len += inc;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len += inc;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len += inc;
            break;
    }
}

/* sdsalloc() = sdsavail() + sdslen() */
// 获取sds分配的内存空间(已用大小+可用大小)
static inline size_t sdsalloc(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->alloc;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->alloc;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->alloc;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->alloc;
    }
    return 0;
}

// 为sds设置分配大小
static inline void sdssetalloc(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            /* Nothing to do, this type has no total allocation info. */
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->alloc = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->alloc = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->alloc = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->alloc = newlen;
            break;
    }
}

// 创建一个sds对象(提供初始值为initlen)
sds sdsnewlen(const void *init, size_t initlen);
// 创建sds对象(没有初始值)
sds sdsnew(const char *init);
// 置空
sds sdsempty(void);
// 复制sds
sds sdsdup(const sds s);
// 释放sds分配的内存
void sdsfree(sds s);
// 如果现在的字符串长度已经是len字节了的话，sdsgrowzero()函数不做任何事情；如果不是，它需要用0字节补齐，把字符串增长到len。
sds sdsgrowzero(sds s, size_t len);
// 字符串连接函数
sds sdscatlen(sds s, const void *t, size_t len); // 定长字符串连接
sds sdscat(sds s, const char *t); // 以空字符结尾
sds sdscatsds(sds s, const sds t); // 连接两个sds字符串
// 字符串复制函数
sds sdscpylen(sds s, const char *t, size_t len); // 定长复制字符串
sds sdscpy(sds s, const char *t);

sds sdscatvprintf(sds s, const char *fmt, va_list ap);
#ifdef __GNUC__
sds sdscatprintf(sds s, const char *fmt, ...)
    __attribute__((format(printf, 2, 3)));
#else
sds sdscatprintf(sds s, const char *fmt, ...);
#endif

sds sdscatfmt(sds s, char const *fmt, ...);
sds sdstrim(sds s, const char *cset);
void sdsrange(sds s, ssize_t start, ssize_t end);
void sdsupdatelen(sds s);
void sdsclear(sds s);
int sdscmp(const sds s1, const sds s2);
sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count);
void sdsfreesplitres(sds *tokens, int count);
void sdstolower(sds s);
void sdstoupper(sds s);
sds sdsfromlonglong(long long value);
sds sdscatrepr(sds s, const char *p, size_t len);
sds *sdssplitargs(const char *line, int *argc);
sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
sds sdsjoin(char **argv, int argc, char *sep);
sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen);

/* Low level functions exposed to the user API */
sds sdsMakeRoomFor(sds s, size_t addlen);
void sdsIncrLen(sds s, ssize_t incr);
sds sdsRemoveFreeSpace(sds s);
size_t sdsAllocSize(sds s);
void *sdsAllocPtr(sds s);

/* Export the allocator used by SDS to the program using SDS.
 * Sometimes the program SDS is linked to, may use a different set of
 * allocators, but may want to allocate or free things that SDS will
 * respectively free or allocate. */
void *sds_malloc(size_t size);
void *sds_realloc(void *ptr, size_t size);
void sds_free(void *ptr);

#ifdef REDIS_TEST
int sdsTest(int argc, char *argv[]);
#endif

#endif

¶sdsalloc.h

/* SDSLib 2.0 -- A C dynamic strings library
 *
 * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
 * Copyright (c) 2015, Redis Labs, Inc
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/* SDS allocator selection.
 *
 * This file is used in order to change the SDS allocator at compile time.
 * Just define the following defines to what you want to use. Also add
 * the include of your alternate allocator if needed (not needed in order
 * to use the default libc allocator). */

/**
 * 再将zmalloc包装为smalloc
 */
#include "zmalloc.h"
#define s_malloc zmalloc
#define s_realloc zrealloc
#define s_free zfree
/* SDSLib 2.0 -- A C dynamic strings library
 *
 * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
 * Copyright (c) 2015, Oran Agra
 * Copyright (c) 2015, Redis Labs, Inc
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */
/**************************************************
 * redis自己构建的简单动态字符串抽象类型，用来代替c语言传统字符串
 * 在redis中，c字符串只作为字面量，sds用来被表示可被修改的字符串
 **************************************************/
#ifndef __SDS_H
#define __SDS_H

// 为sds预留的最大分配内存
#define SDS_MAX_PREALLOC (1024*1024)
const char *SDS_NOINIT;

#include <sys/types.h>
#include <stdarg.h>
#include <stdint.h>

// 将char* 指针自定义为sds类型
typedef char *sds;

// __attribute__ ((__packed__))关键字 https://blog.csdn.net/weixin_39533180/article/details/76207099
// 功能： 结构体建立的时候，会进行字节对齐。为了减少内存占用，使用这个关键字取消字节对齐，按照紧凑排列的方式，占用内存

/* Note: sdshdr5 is never used, we just access the flags byte directly.
 * However is here to document the layout of type 5 SDS strings. */
/**
 * lsb(Linux Standards Base/ least significant bit)和msb(Most Significant Bit)
 * 参考：https://zhidao.baidu.com/question/155072477.html
 * lsb: 最低有效位,msb: 最高有效位,MSB位于二进制数的最左侧，LSB位于二进制数的最右侧。
 *
 * sdshdr使用场景分析 : https://www.codercto.com/a/46648.html
 */

/* sds的五种类型
 * len : sds的长度，不包括结尾结束符
 * alloc : 分配的sds长度,不包括结束符
 * flag: sds类型
 * buf: sds实际存放位置
 */
// flag低三位保存sds类型，flag高三位保存string长度(buf)
struct __attribute__ ((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char buf[];
};
// uint8_t: 无符号char (-128~127) char默认是 signed char
struct __attribute__ ((__packed__)) sdshdr8 {
    uint8_t len; /* used */
    uint8_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint16_t : short int (占2字节)
struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint32_t : int (占4字节)
struct __attribute__ ((__packed__)) sdshdr32 {
    uint32_t len; /* used */
    uint32_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
// uint64_t : long int (占8字节)
struct __attribute__ ((__packed__)) sdshdr64 {
    uint64_t len; /* used */
    uint64_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};

// 为sds类型定义标志码
#define SDS_TYPE_5  0
#define SDS_TYPE_8  1
#define SDS_TYPE_16 2
#define SDS_TYPE_32 3
#define SDS_TYPE_64 4
#define SDS_TYPE_MASK 7
#define SDS_TYPE_BITS 3

/**
 * 这三个函数共通的地方就是功能都是为了去除sds类型和结构，获得有效的数据
 *
 * SDS_HDR_VAR(T,s)/SDS_HDR(T,s)
 * T传入的是(5,8,16,32,64),sdshdr##T拼接为sdshdr5,sdshdr8,sdshdr16,sdshdr32,sdshdr64
 * s传入的是字符串
 * (void*)((s)-(sizeof(struct sdshdr##T))) 是为了将字符串的sds结构体所占大小减去，获得字符串实际长度
 *
 * SDS_HDR(T,s)和SDS_HDR_VAR(T,s)区别就是SDS_HDR_VAR(T,s)是需要变量中转，SDS_HDR(T,s)不需要变量中转
 */
#define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
// f 是传入的flag,(f)>>3表示将3位最低有效位(存储sds类型)消除,获得没有类型的长度
#define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)

// 获取sds长度
static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    // flag存储的是sds五种类型的二进制 5(0000),8(0001),16(0010),32(0011),64(0100)
    // flag&SDS_TYPE_MASK = 0000 & 0111 = 0000 ; 最终得到的值就是flag。SDS_TYPE_MASK是屏蔽位，不需要检查的位置0，检查到对应的位对应置1
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}

// 获取sds可用字节数(分配长度-已经分配的长度)
static inline size_t sdsavail(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: {
            return 0;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            return sh->alloc - sh->len;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            return sh->alloc - sh->len;
        }
    }
    return 0;
}

// 为sds设置长度
static inline void sdssetlen(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: // sdshdr5没有len属性，不能直接给其赋值,需要按相反的操作来
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS); // 加上最低有效位(sds类型)
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len = newlen;
            break;
    }
}

// sds在原有基础上增加inc的长度
static inline void sdsinclen(sds s, size_t inc) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            {
                unsigned char *fp = ((unsigned char*)s)-1;
                unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
                *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
            }
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->len += inc;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->len += inc;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->len += inc;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->len += inc;
            break;
    }
}

/* sdsalloc() = sdsavail() + sdslen() */
// 获取sds分配的内存空间(已用大小+可用大小)
static inline size_t sdsalloc(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->alloc;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->alloc;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->alloc;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->alloc;
    }
    return 0;
}

// 为sds设置分配大小
static inline void sdssetalloc(sds s, size_t newlen) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            /* Nothing to do, this type has no total allocation info. */
            break;
        case SDS_TYPE_8:
            SDS_HDR(8,s)->alloc = newlen;
            break;
        case SDS_TYPE_16:
            SDS_HDR(16,s)->alloc = newlen;
            break;
        case SDS_TYPE_32:
            SDS_HDR(32,s)->alloc = newlen;
            break;
        case SDS_TYPE_64:
            SDS_HDR(64,s)->alloc = newlen;
            break;
    }
}

// 创建一个sds对象(提供初始值为initlen)
sds sdsnewlen(const void *init, size_t initlen);
// 创建sds对象(没有初始值)
sds sdsnew(const char *init);
// 置空
sds sdsempty(void);
// 复制sds
sds sdsdup(const sds s);
// 释放sds分配的内存
void sdsfree(sds s);
// 如果现在的字符串长度已经是len字节了的话，sdsgrowzero()函数不做任何事情；如果不是，它需要用0字节补齐，把字符串增长到len。
sds sdsgrowzero(sds s, size_t len);
// 字符串连接函数
sds sdscatlen(sds s, const void *t, size_t len); // 定长字符串连接
sds sdscat(sds s, const char *t); // 以空字符结尾
sds sdscatsds(sds s, const sds t); // 连接两个sds字符串
// 字符串复制函数
sds sdscpylen(sds s, const char *t, size_t len); // 定长复制字符串
sds sdscpy(sds s, const char *t);

sds sdscatvprintf(sds s, const char *fmt, va_list ap);
#ifdef __GNUC__
sds sdscatprintf(sds s, const char *fmt, ...)
    __attribute__((format(printf, 2, 3)));
#else
sds sdscatprintf(sds s, const char *fmt, ...);
#endif

sds sdscatfmt(sds s, char const *fmt, ...);
sds sdstrim(sds s, const char *cset);
void sdsrange(sds s, ssize_t start, ssize_t end);
void sdsupdatelen(sds s);
void sdsclear(sds s);
int sdscmp(const sds s1, const sds s2);
sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count);
void sdsfreesplitres(sds *tokens, int count);
void sdstolower(sds s);
void sdstoupper(sds s);
sds sdsfromlonglong(long long value);
sds sdscatrepr(sds s, const char *p, size_t len);
sds *sdssplitargs(const char *line, int *argc);
sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
sds sdsjoin(char **argv, int argc, char *sep);
sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen);

/* Low level functions exposed to the user API */
sds sdsMakeRoomFor(sds s, size_t addlen);
void sdsIncrLen(sds s, ssize_t incr);
sds sdsRemoveFreeSpace(sds s);
size_t sdsAllocSize(sds s);
void *sdsAllocPtr(sds s);

/* Export the allocator used by SDS to the program using SDS.
 * Sometimes the program SDS is linked to, may use a different set of
 * allocators, but may want to allocate or free things that SDS will
 * respectively free or allocate. */
void *sds_malloc(size_t size);
void *sds_realloc(void *ptr, size_t size);
void sds_free(void *ptr);

#ifdef REDIS_TEST
int sdsTest(int argc, char *argv[]);
#endif

#endif

¶sds.c

/* SDSLib 2.0 -- A C dynamic strings library
 *
 * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
 * Copyright (c) 2015, Oran Agra
 * Copyright (c) 2015, Redis Labs, Inc
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <assert.h>
#include <limits.h>
#include "sds.h"
#include "sdsalloc.h"

const char *SDS_NOINIT = "SDS_NOINIT";

// 引用sds.h中的SDS_TYPE_系列定义
// 返回各个sds类型的结构体大小
static inline int sdsHdrSize(char type) {
    switch(type&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return sizeof(struct sdshdr5);
        case SDS_TYPE_8:
            return sizeof(struct sdshdr8);
        case SDS_TYPE_16:
            return sizeof(struct sdshdr16);
        case SDS_TYPE_32:
            return sizeof(struct sdshdr32);
        case SDS_TYPE_64:
            return sizeof(struct sdshdr64);
    }
    return 0;
}

// 通过字符串大小判断该字符串属于哪种sds类型。如果字符串大小小于sds其中一个类型的最大范围，那这个字符串就是这个sds类型的字符串
static inline char sdsReqType(size_t string_size) {
    if (string_size < 1<<5) // 1<<5表示sdshdr5类型字符串最大范围
        return SDS_TYPE_5;
    if (string_size < 1<<8)
        return SDS_TYPE_8;
    if (string_size < 1<<16)
        return SDS_TYPE_16;
#if (LONG_MAX == LLONG_MAX)
    if (string_size < 1ll<<32)
        return SDS_TYPE_32;
    return SDS_TYPE_64;
#else
    return SDS_TYPE_32;
#endif
}

/* Create a new sds string with the content specified by the 'init' pointer
 * and 'initlen'.
 * If NULL is used for 'init' the string is initialized with zero bytes.
 * If SDS_NOINIT is used, the buffer is left uninitialized;
 *
 * The string is always null-termined (all the sds strings are, always) so
 * even if you create an sds string with:
 *
 * mystring = sdsnewlen("abc",3);
 *
 * You can print the string with printf() as there is an implicit \0 at the
 * end of the string. However the string is binary safe and can contain
 * \0 characters in the middle, as the length is stored in the sds header. */
// 创建一个initlen长度的sds对象
/* 创建一个sds字符串的流程
 * 确定sds类型
 * 分配内存
 *
 */
sds sdsnewlen(const void *init, size_t initlen) {
    void *sh;
    sds s;
    char type = sdsReqType(initlen); // 通过字符串长度判断符合的sds类型
    /* Empty strings are usually created in order to append. Use type 8
     * since type 5 is not good at this. */
    // redis现在已经不再使用sdshdr5类型了
    if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
    int hdrlen = sdsHdrSize(type); // 通过sds类型给initlen加上结构体长度,形成一个完整的sds对象需要的长度
    unsigned char *fp; /* flags pointer. */ //类型指针

    sh = s_malloc(hdrlen+initlen+1); // 为sds对象分配内存,+1是将空字符也算进去
    if (init==SDS_NOINIT) // 如果sds没有被初始化，则分配大小为0
        init = NULL;
    else if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    if (sh == NULL) return NULL;
    // 指针sh已经分配了hdrlen+initlen+1的空间，现在指针s指向指针sh指向的这个空间中的hdrlen空间部分，就相当于给指针s分配了hdrlen的空间
    s = (char*)sh+hdrlen;
    fp = ((unsigned char*)s)-1; // ((unsigned char*)s)-1表示从指针s中划一个字节，分配给指针fp,用来表示sds类型
    switch(type) { // 判断类型 , 然后为s的各个属性初始化
        case SDS_TYPE_5: {
            *fp = type | (initlen << SDS_TYPE_BITS);
            break;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
    }
    if (initlen && init) // initlen和init都不为空
        memcpy(s, init, initlen); // 将initlen长度的字符串init复制到s的内存区域,这个直接在内存上操作，不用考虑分配内存
        // void *memcpy(void *dest, const void *src, size_t n);
        // 将指针src指向的前initlen字节到dest指定的内存地址上
    s[initlen] = '\0'; // 最终以空字符结尾，这表示一个完整的字符串
    return s;
}

/* Create an empty (zero length) sds string. Even in this case the string
 * always has an implicit null term. */
// 使用sdsnewlen创建一个空的sds字符串
sds sdsempty(void) {
    return sdsnewlen("",0);
}

/* Create a new sds string starting from a null terminated C string. */
sds sdsnew(const char *init) {
    size_t initlen = (init == NULL) ? 0 : strlen(init);
    return sdsnewlen(init, initlen);
}

/* Duplicate an sds string. */
// 复制字符串
sds sdsdup(const sds s) {
    return sdsnewlen(s, sdslen(s));
}

/* Free an sds string. No operation is performed if 's' is NULL. */
// 释放分配给字符串的内存
void sdsfree(sds s) {
    if (s == NULL) return;
    s_free((char*)s-sdsHdrSize(s[-1]));
}

/* Set the sds string length to the length as obtained with strlen(), so
 * considering as content only up to the first null term character.
 *
 * This function is useful when the sds string is hacked manually in some
 * way, like in the following example:
 *
 * s = sdsnew("foobar");
 * s[2] = '\0';
 * sdsupdatelen(s);
 * printf("%d\n", sdslen(s));
 * 最终结果是 2 ； 如果去掉sdsupdatelen(s);字符串的逻辑长度是6.
 * 虽然字符串已经在'\0'结束，但是没有使用sdsupdatelen重新设置长度，s的'\0'的后面的字符还存在，但是没有被显示。sdsupdatelen可以将后面不显示的字符串去掉
 * The output will be "2", but if we comment out the call to sdsupdatelen()
 * the output will be "6" as the string was modified but the logical length
 * remains 6 bytes. */
// 重新为字符串设置长度
void sdsupdatelen(sds s) {
    size_t reallen = strlen(s);
    sdssetlen(s, reallen);
}

/* Modify an sds string in-place to make it empty (zero length).
 * However all the existing buffer is not discarded but set as free space
 * so that next append operations will not require allocations up to the
 * number of bytes previously available. */
//将sds长度置0
void sdsclear(sds s) {
    sdssetlen(s, 0);
    s[0] = '\0';
}

/* Enlarge the free space at the end of the sds string so that the caller
 * is sure that after calling this function can overwrite up to addlen
 * bytes after the end of the string, plus one more byte for nul term.
 *
 * Note: this does not change the *length* of the sds string as returned
 * by sdslen(), but only the free buffer space we have. */
// 增加sds的长度(增加长度的前提是addlen比sds可用空间大，不然不用增加长度)
sds sdsMakeRoomFor(sds s, size_t addlen) {
    void *sh, *newsh;
    size_t avail = sdsavail(s);// 获取可用字节数
    size_t len, newlen;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;

    /* Return ASAP if there is enough space left. */
    if (avail >= addlen) return s; // 如果原本字符串的可用字节数比增加的长度大，直接就返回这个字符串，不需要增加长度

    len = sdslen(s); // 获sds字符串s的真实长度(去掉结构所占长度)
    sh = (char*)s-sdsHdrSize(oldtype); // 这里让指针sh指向纯粹字符串(减去字符串类型的长度的字符串)
    newlen = (len+addlen); // 这里得到新的字符串的长度
    if (newlen < SDS_MAX_PREALLOC) // 如果新分配的字符串长度小于sds预分配的长度
        newlen *= 2; // 就增加一倍
    else
        newlen += SDS_MAX_PREALLOC; // 如果大于，就再在新长度基础上加上sds预分配大小

    type = sdsReqType(newlen); // 通过新长度获取sds字符串匹配的类型

    /* Don't use type 5: the user is appending to the string and type 5 is
     * not able to remember empty space, so sdsMakeRoomFor() must be called
     * at every appending operation. */
    // 这里抛弃sdshdr5
    if (type == SDS_TYPE_5) type = SDS_TYPE_8;

    hdrlen = sdsHdrSize(type); // 获取新类型的类型长度
    if (oldtype==type) { // 如果新类型还是和旧类型一样
        newsh = s_realloc(sh, hdrlen+newlen+1); // 指针指向的内存区块不变，在这个区块之后再分配新长度，直到总长度与hdrlen+newlen+1相等
        if (newsh == NULL) return NULL; // 如果新分配为空
        s = (char*)newsh+hdrlen; // 这里先把指针s指向hdrlen位置(将hdrlen分配给了s)
    } else {
        /* Since the header size changes, need to move the string forward,
         * and can't use realloc */
        // 这里只能重新分配内存
        newsh = s_malloc(hdrlen+newlen+1);
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1); //newsh先分配hdrlen长度的内存存储类型，然后将len+1长度的内存用来存放s,之外还有新增的空闲长度
        s_free(sh); // 然后释放sh内存
        s = (char*)newsh+hdrlen; // s指针指向指针newsh指向的内存区域，并将hdrlen长度的内存分配给类型
        s[-1] = type;
        sdssetlen(s, len);// 然后给s设置被用的长度
    }
    sdssetalloc(s, newlen); // 重新给指针s设置分配的长度
    return s; // 现在返回的字符串s指向的内存区域已经重新被分配
}

/* Reallocate the sds string so that it has no free space at the end. The
 * contained string remains not altered, but next concatenation operations
 * will require a reallocation.
 *
 * After the call, the passed sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
// 移除空闲空间
sds sdsRemoveFreeSpace(sds s) {
    void *sh, *newsh;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
    size_t len = sdslen(s);// 获得该字符串已用长度
    size_t avail = sdsavail(s); // 获取可用长度
    sh = (char*)s-oldhdrlen;//指向剔除类型长度的字符串

    /* Return ASAP if there is no space left. */
    if (avail == 0) return s; // 如果可用长度为0，表示该字符串没有需要释放的空闲空间

    /* Check what would be the minimum SDS header that is just good enough to
     * fit this string. */
    type = sdsReqType(len); // 通过字符串已用长度获取字符串类型
    hdrlen = sdsHdrSize(type); // 获取类型长度

    /* If the type is the same, or at least a large enough type is still
     * required, we just realloc(), letting the allocator to do the copy
     * only if really needed. Otherwise if the change is huge, we manually
     * reallocate the string to use the different header type. */
    if (oldtype==type || type > SDS_TYPE_8) { // 如果新类型(剔除空闲长度后的字符串类型)与旧类型相等(包括空闲长度的字符串类型) 或者新类型不是sdshdr5
        newsh = s_realloc(sh, oldhdrlen+len+1); // 直接在原来的内存基础上缩小为len+1大小的内存
        if (newsh == NULL) return NULL;
        s = (char*)newsh+oldhdrlen; // 指针sh指向newsh指针指向的地址，并将指针移动到字符串类型长度那里，使得字符串从字符串类型结束开始存入
    } else {
        newsh = s_malloc(hdrlen+len+1); // 如果新类型不等于旧类型，就需要重新分配内存
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1); // 将字符串s的内存区域复制给从hdrlen地址开始，长度为len+1的内存区域
        s_free(sh); // 释放中间指针变量sh内存
        s = (char*)newsh+hdrlen; // 让指针s指向指针newsh，并将指针游标偏移hdrlen长度，从这里开始存储字符串
        s[-1] = type; // 在s[-1]存储字符串类型
        sdssetlen(s, len); // 给s设置被用长度
    }
    sdssetalloc(s, len); // 给s设置分配的长度为len(与被用长度一样)，表示空闲空间已被释放
    return s;
}

/* Return the total size of the allocation of the specified sds string,
 * including:
 * 1) The sds header before the pointer.
 * 2) The string.
 * 3) The free buffer at the end if any.
 * 4) The implicit null term.
 */
 // 获取分配的长度
size_t sdsAllocSize(sds s) {
    size_t alloc = sdsalloc(s); // 获取分配的长度(未包含字符串类型长度和结尾空字符)
    return sdsHdrSize(s[-1])+alloc+1;
}

/* Return the pointer of the actual SDS allocation (normally SDS strings
 * are referenced by the start of the string buffer). */
// 获取指向分配给字符串的内存的指针
void *sdsAllocPtr(sds s) {
    return (void*) (s-sdsHdrSize(s[-1]));
}

/* Increment the sds length and decrements the left free space at the
 * end of the string according to 'incr'. Also set the null term
 * in the new end of the string.
 *
 * This function is used in order to fix the string length after the
 * user calls sdsMakeRoomFor(), writes something after the end of
 * the current string, and finally needs to set the new length.
 *
 * Note: it is possible to use a negative increment in order to
 * right-trim the string.
 *
 * Usage example:
 *
 * Using sdsIncrLen() and sdsMakeRoomFor() it is possible to mount the
 * following schema, to cat bytes coming from the kernel to the end of an
 * sds string without copying into an intermediate buffer:
 *
 * oldlen = sdslen(s);
 * s = sdsMakeRoomFor(s, BUFFER_SIZE);
 * nread = read(fd, s+oldlen, BUFFER_SIZE);
 * ... check for nread <= 0 and handle it ...
 * sdsIncrLen(s, nread);
 */
// 增加sds长度
void sdsIncrLen(sds s, ssize_t incr) {
    unsigned char flags = s[-1];
    size_t len;
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5: {
            unsigned char *fp = ((unsigned char*)s)-1;
            unsigned char oldlen = SDS_TYPE_5_LEN(flags);
            assert((incr > 0 && oldlen+incr < 32) || (incr < 0 && oldlen >= (unsigned int)(-incr)));
            *fp = SDS_TYPE_5 | ((oldlen+incr) << SDS_TYPE_BITS);
            len = oldlen+incr;
            break;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
            len = (sh->len += incr);
            break;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
            len = (sh->len += incr);
            break;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            assert((incr >= 0 && sh->alloc-sh->len >= (unsigned int)incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
            len = (sh->len += incr);
            break;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            assert((incr >= 0 && sh->alloc-sh->len >= (uint64_t)incr) || (incr < 0 && sh->len >= (uint64_t)(-incr)));
            len = (sh->len += incr);
            break;
        }
        default: len = 0; /* Just to avoid compilation warnings. */
    }
    s[len] = '\0';
}

/* Grow the sds to have the specified length. Bytes that were not part of
 * the original length of the sds will be set to zero.
 *
 * if the specified length is smaller than the current length, no operation
 * is performed. */
// 为sds的空闲空间置0
sds sdsgrowzero(sds s, size_t len) {
    size_t curlen = sdslen(s); // 获取sds当前被用的字符长度

    if (len <= curlen) return s; // 如果输入的长度小于当前长度，直接就返回s
    s = sdsMakeRoomFor(s,len-curlen); // 如果len-curlen长度小于s的空闲长度，则直接返回s;如果大于，就为s增加长度(s的长度就是被用空间+(len-curlen))
    if (s == NULL) return NULL;

    /* Make sure added region doesn't contain garbage */
    // 确保增加的内存不包含垃圾
    memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */
    sdssetlen(s, len); // 将输入的长度设置为新长度
    return s;
}

/* Append the specified binary-safe string pointed by 't' of 'len' bytes to the
 * end of the specified sds string 's'.
 *
 * After the call, the passed sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
// 将一个字符串与一个空指针类型连接(自定义长度)
sds sdscatlen(sds s, const void *t, size_t len) {
    size_t curlen = sdslen(s); // s字符串的被用长度

    s = sdsMakeRoomFor(s,len); // 为字符串s扩充len长度
    if (s == NULL) return NULL;
    memcpy(s+curlen, t, len); // 将长度为len的t的内容复制到指针s在curlen地址之后的地址
    sdssetlen(s, curlen+len); // 为新字符串设置新长度
    s[curlen+len] = '\0'; // 空字符结尾
    return s;
}

/* Append the specified null termianted C string to the sds string 's'.
 *
 * After the call, the passed sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
// 封装了sdscatlen函数，为其len设置默认值
sds sdscat(sds s, const char *t) {
    return sdscatlen(s, t, strlen(t));
}

/* Append the specified sds 't' to the existing sds 's'.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
// 将两个sds类型连接(空类型被强转为sds类型)
sds sdscatsds(sds s, const sds t) {
    return sdscatlen(s, t, sdslen(t));
}

/* Destructively modify the sds string 's' to hold the specified binary
 * safe string pointed by 't' of length 'len' bytes. */
// 复制sds
sds sdscpylen(sds s, const char *t, size_t len) {
    if (sdsalloc(s) < len) { // 如果sds分配的空间小于len,表示len长度的内存能接受复制内容
        s = sdsMakeRoomFor(s,len-sdslen(s)); // 将s扩充为len长度,使得两个字符串长度一样
        if (s == NULL) return NULL;
    }
    memcpy(s, t, len); // 内存复制
    s[len] = '\0';
    sdssetlen(s, len); // 为s重新设置长度
    return s;
}

/* Like sdscpylen() but 't' must be a null-termined string so that the length
 * of the string is obtained with strlen(). */
// 封装sdscpylen
sds sdscpy(sds s, const char *t) {
    return sdscpylen(s, t, strlen(t));
}

/* Helper for sdscatlonglong() doing the actual number -> string
 * conversion. 's' must point to a string with room for at least
 * SDS_LLSTR_SIZE bytes.
 *
 * The function returns the length of the null-terminated string
 * representation stored at 's'. */
#define SDS_LLSTR_SIZE 21
/*
	数值转换，将long long类型的整型转换为字符串,并以'\0'结尾。最终将字符串存放在s中,返回转换后的长度
	函数实现流程：
		1. 将数值从右到左一位一位转换为字符，存放到指针中
		2. 如果是负数，加上'-' 
		3. 算出长度，结尾加上'\0'
		4. 把字符串反转，使得数值转换成的字符串是从左到右的
*/
int sdsll2str(char *s, long long value) {
    char *p, aux;
    unsigned long long v;
    size_t l;

    /* Generate the string representation, this method produces
     * an reversed string. */
    v = (value < 0) ? -value : value; // 让value一直是正数
    p = s; // 指针p指向字符串
    do {
        *p++ = '0'+(v%10); // 将整数v的每一位转换为字符串放入指针p中
        v /= 10;
    } while(v);
    if (value < 0) *p++ = '-'; // 如果value小于0,则在p的新的地址的内容中存放-,表示为负数

    /* Compute length and add null term. */
    // 计算长度和增加结尾空字符
    l = p-s;
    *p = '\0';

    /* Reverse the string. */
    // 反转字符串
    p--;
    while(s < p) {
        aux = *s;
        *s = *p;
        *p = aux;
        s++;
        p--;
    }
    return l;
}

/* Identical sdsll2str(), but for unsigned long long type. */
// 功能与上面函数差不多，只是转换的数值v变成无符号类型，不用考虑是否是正数了
int sdsull2str(char *s, unsigned long long v) {
    char *p, aux;
    size_t l;

    /* Generate the string representation, this method produces
     * an reversed string. */
    p = s;
    do {
        *p++ = '0'+(v%10);
        v /= 10;
    } while(v);

    /* Compute length and add null term. */
    l = p-s;
    *p = '\0';

    /* Reverse the string. */
    p--;
    while(s < p) {
        aux = *s;
        *s = *p;
        *p = aux;
        s++;
        p--;
    }
    return l;
}

/* Create an sds string from a long long value. It is much faster than:
 *
 * sdscatprintf(sdsempty(),"%lld\n", value);
 */
 // 从long long value数值创建为一个sds字符串
sds sdsfromlonglong(long long value) {
    char buf[SDS_LLSTR_SIZE];
    int len = sdsll2str(buf,value); // 先将数值转换为字符串，存储到buf中,并获取它的长度

    return sdsnewlen(buf,len); // 通过长度和字符串，创建为一个sds字符串
}

/* Like sdscatprintf() but gets va_list instead of being variadic. */
/*
	将字符串s与格式化字符串连接，最终返回连接后的新sds字符串
	a="f"
	t = sdscatvprintf("abc" , "de%s",a)
	t = "abcdef"
*/

sds sdscatvprintf(sds s, const char *fmt, va_list ap) {
    va_list cpy;
    char staticbuf[1024], *buf = staticbuf, *t;
    size_t buflen = strlen(fmt)*2;

    /* We try to start using a static buffer for speed.
     * If not possible we revert to heap allocation. */
     // 哪个大就使用哪个
    if (buflen > sizeof(staticbuf)) { // 如果fmt长度的2倍大于字符数组的大小
        buf = s_malloc(buflen); // 为buf分配buflen长度的内存
        if (buf == NULL) return NULL;
    } else { // 将buflen重置为staticbuf的大小,并且buf指向staticbuf
        buflen = sizeof(staticbuf);
    }

    /* Try with buffers two times bigger every time we fail to
     * fit the string in the current buffer size. */
    while(1) {
        buf[buflen-2] = '\0'; // 将'\n'清除
        va_copy(cpy,ap); // 将ap的参数列表复制给cpy
        vsnprintf(buf, buflen, fmt, cpy); // 将buflen长度的字符串(cpy的参数列表的值被放入fmt格式中，形成一个字符串)输出到buf字符串中
        va_end(cpy); // cpy参数列表被消除
        if (buf[buflen-2] != '\0') { // 如果不是'\0'表示复制给buf的字符太多了，已经占用了buflen-2位置。这时就需要先释放buf内存，为buf扩容，重新分配内存，重新循环一次。直到buf的容量可以满足fmt字符串
            if (buf != staticbuf) s_free(buf);
            buflen *= 2;
            buf = s_malloc(buflen);
            if (buf == NULL) return NULL;
            continue;
        }
        break;
    }

    /* Finally concat the obtained string to the SDS string and return it. */
    t = sdscat(s, buf); // 将字符s与字符串buf连接为字符串t
    if (buf != staticbuf) s_free(buf);
    return t;
}

/* Append to the sds string 's' a string obtained using printf-alike format
 * specifier.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call.
 *
 * Example:
 *
 * s = sdsnew("Sum is: ");
 * s = sdscatprintf(s,"%d+%d = %d",a,b,a+b).
 *
 * Often you need to create a string from scratch with the printf-alike
 * format. When this is the need, just use sdsempty() as the target string:
 *
 * s = sdscatprintf(sdsempty(), "... your format ...", args);
 */
 // 封装了sdscatvprintf函数
sds sdscatprintf(sds s, const char *fmt, ...) {
    va_list ap;
    char *t;
    va_start(ap, fmt); // 参数列表ap以fmt的类型为模板
    t = sdscatvprintf(s,fmt,ap);
    va_end(ap);
    return t;
}

/* This function is similar to sdscatprintf, but much faster as it does
 * not rely on sprintf() family functions implemented by the libc that
 * are often very slow. Moreover directly handling the sds string as
 * new data is concatenated provides a performance improvement.
 *
 * However this function only handles an incompatible subset of printf-alike
 * format specifiers:
 *
 * %s - C String
 * %S - SDS string
 * %i - signed int
 * %I - 64 bit signed integer (long long, int64_t)
 * %u - unsigned int
 * %U - 64 bit unsigned integer (unsigned long long, uint64_t)
 * %% - Verbatim "%" character.
 */
 /* 这个函数与sdscatprintf函数相似，但是它不会将字符串打印出来
  * 
  * sdscatprintf函数依托了vsnprintf函数将fmt格式化，但是这个函数没有引用prinf，因此需要自己实现格式化功能
  */
sds sdscatfmt(sds s, char const *fmt, ...) {
    size_t initlen = sdslen(s); // 获取s字符串长度
    const char *f = fmt; 
    long i;
    va_list ap;

    va_start(ap,fmt);
    f = fmt;    /* Next format specifier byte to process. */
    i = initlen; /* Position of the next byte to write to dest str. */
    while(*f) { // 字符串的第一个字符，确定字符串f是否为空
        char next, *str;
        size_t l;
        long long num;
        unsigned long long unum;

        /* Make sure there is always space for at least 1 char. */
        if (sdsavail(s)==0) { // 确保s字符串空闲空间最少为l个字符
            s = sdsMakeRoomFor(s,1);
        }

        switch(*f) { // 现在f指针指向的字符
        case '%': // 如果是%,表示有格式化数据需要格式化,那其后面就是需要格式化的类型
            next = *(f+1);
            f++;
            switch(next) { // 看这个格式化类型是什么(s,d,ld,f等等)
            case 's': // 忽略字符大小写
            case 'S':
                str = va_arg(ap,char*); // 将参数列表的参数的值转换为字符串(参数列表是通过指针读取，如果读取一个参数，指针向后偏移，下一次就是读取下一个参数)
                l = (next == 's') ? strlen(str) : sdslen(str); // 如果是's'(c内置字符串),直接用strlen获取长度;如果是'S'(sds字符串),使用sdslen获取长度
                if (sdsavail(s) < l) {
                    s = sdsMakeRoomFor(s,l); // 为s增加可以容纳输入参数字符串长度的空间
                }
                memcpy(s+i,str,l); // 将长度为l的参数的值复制到字符串的后面
                sdsinclen(s,l); // 为字符串s增加长度
                i += l; // 相应的游标也需要增加l长度
                break;
            case 'i': // 如果是有符号整型
            case 'I':
                if (next == 'i') // 如果是c内置整型
                    num = va_arg(ap,int); // 获取当前指针指向的参数列表中的参数，将其值转换为整型
                else
                    num = va_arg(ap,long long); // 否则是redis包装的整型，将其转换为long long类型
                // 代码块中的代码作用域只是这个代码块
                {
                    char buf[SDS_LLSTR_SIZE];
                    l = sdsll2str(buf,num); // 将num转换为字符串，并返回其长度
                    if (sdsavail(s) < l) {
                        s = sdsMakeRoomFor(s,l); // 为s增加l的长度
                    }
                    memcpy(s+i,buf,l); // 将长度为l的字符串buf复制到s的后面
                    sdsinclen(s,l); // s长度增加l
                    i += l; // 游标向后移动l长度
                }
                break;
            case 'u': // 如果是无符号整型,原理和上面的整型一样
            case 'U':
                if (next == 'u')
                    unum = va_arg(ap,unsigned int);
                else
                    unum = va_arg(ap,unsigned long long);
                {
                    char buf[SDS_LLSTR_SIZE];
                    l = sdsull2str(buf,unum);
                    if (sdsavail(s) < l) {
                        s = sdsMakeRoomFor(s,l);
                    }
                    memcpy(s+i,buf,l);
                    sdsinclen(s,l);
                    i += l;
                }
                break;
            default: /* Handle %% and generally %<unknown>. */
                s[i++] = next; // 如果格式化类型是"%",表示是不知道的类型，直接将其加入s后面就行了 
                sdsinclen(s,1); // s的长度相应的增加
                break;
            }
            break;
        default:
            s[i++] = *f; // 如果没有遇到格式化i标志"%"，那就直接将字符加入到s的后面
            sdsinclen(s,1);// 并将s的长度增加l
            break;
        }
        f++;
    }
    va_end(ap);

    /* Add null-term */
    s[i] = '\0'; // 操作完之后为字符串结尾增加空字符表示一个完整的字符串
    return s;
}

/* Remove the part of the string from left and from right composed just of
 * contiguous characters found in 'cset', that is a null terminted C string.
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call.
 *
 * Example:
 *
 * s = sdsnew("AA...AA.a.aa.aHelloWorld     :::");
 * s = sdstrim(s,"Aa. :");
 * printf("%s\n", s);
 *
 * Output will be just "HelloWorld".
 */
// 移除字符串s的左右指定字符(cset用来进行模式匹配)
sds sdstrim(sds s, const char *cset) {
    char *start, *end, *sp, *ep;
    size_t len;

    sp = start = s;
    ep = end = s+sdslen(s)-1;
    // 当指向开始的指针sp指向的内存地址没有超过指向结尾的end指针指向的内存地址，并且指针sp指向地址的内容在cset字符串中有匹配的字符，那么执行开始的指针sp的指针向后偏移一位
    while(sp <= end && strchr(cset, *sp)) sp++;
    while(ep > sp && strchr(cset, *ep)) ep--; // 与上面相反，向前偏移
    len = (sp > ep) ? 0 : ((ep-sp)+1); // 当指向开始的指针sp指向的地址大于指向结尾的指针end指向的地址，表示没有被cset字符串中字符不匹配的字符，那么需要清除就是整个字符串，字符串长度就归0;如果相反，则还有剩余的字符没有被cset匹配，那就输出这些没有匹配的字符串以及其长度
    if (s != sp) memmove(s, sp, len); // 如果当前字符串指针指向的地址不是sp指针指向的地址，则将以sp指针指向的地址为开始的长度为len的内存区域移动到以指针s指向的地址为开始的内存区
    s[len] = '\0';// 字符串以空字符结尾
    sdssetlen(s,len); // 为字符串设置长度
    return s;
}

/* Turn the string into a smaller (or equal) string containing only the
 * substring specified by the 'start' and 'end' indexes.
 *
 * start and end can be negative, where -1 means the last character of the
 * string, -2 the penultimate character, and so forth.
 *
 * The interval is inclusive, so the start and end characters will be part
 * of the resulting string.
 *
 * The string is modified in-place.
 *
 * Example:
 *
 * s = sdsnew("Hello World");
 * sdsrange(s,1,-1); => "ello World"
 */
 // 将字符串s中以start为开始位置，以end为结束位置的范围的字符切出来作为子串，重新赋值给s
void sdsrange(sds s, ssize_t start, ssize_t end) {
    size_t newlen, len = sdslen(s); // 先获取sds字符串的被用长度

    if (len == 0) return; // 如果字符串长度为0
    
    //将越界的索引放入索引范围之内
    if (start < 0) {
        start = len+start; 
        if (start < 0) start = 0;
    }
    if (end < 0) {
        end = len+end;
        if (end < 0) end = 0;
    }
    
    // 开始索引大于结束索引，就将新字符串长度置0;否则获取新字符串长度
    newlen = (start > end) ? 0 : (end-start)+1;
    if (newlen != 0) {
		// 这里防止索引不在字符串长度范围内;如果不在，就强制让超出范围的索引包围整个字符串(start=0,end=len-1)
        if (start >= (ssize_t)len) {
            newlen = 0;
        } else if (end >= (ssize_t)len) {
            end = len-1;
            newlen = (start > end) ? 0 : (end-start)+1;
        }
    } else { // 如果新长度为0,则开始索引置0
        start = 0;
    }
    // 如果开始索引不为0,并且新长度不为0
    if (start && newlen) memmove(s, s+start, newlen);
    s[newlen] = 0;
    sdssetlen(s,newlen);
}

/* Apply tolower() to every character of the sds string 's'. */
// 字符串转小写,使用tolower函数
void sdstolower(sds s) {
    size_t len = sdslen(s), j;

    for (j = 0; j < len; j++) s[j] = tolower(s[j]);
}

/* Apply toupper() to every character of the sds string 's'. */
// 字符串转大写，应用toupper函数
void sdstoupper(sds s) {
    size_t len = sdslen(s), j;

    for (j = 0; j < len; j++) s[j] = toupper(s[j]);
}

/* Compare two sds strings s1 and s2 with memcmp().
 *
 * Return value:
 *
 *     positive if s1 > s2.
 *     negative if s1 < s2.
 *     0 if s1 and s2 are exactly the same binary string.
 *
 * If two strings share exactly the same prefix, but one of the two has
 * additional characters, the longer string is considered to be greater than
 * the smaller one. */
 // 两个字符串之间比较是否相同
int sdscmp(const sds s1, const sds s2) {
    size_t l1, l2, minlen;
    int cmp;

    l1 = sdslen(s1);
    l2 = sdslen(s2);
    minlen = (l1 < l2) ? l1 : l2;
    cmp = memcmp(s1,s2,minlen); // 比较的是内存区域
    if (cmp == 0) return l1>l2? 1: (l1<l2? -1: 0);
    return cmp;
}

/* Split 's' with separator in 'sep'. An array
 * of sds strings is returned. *count will be set
 * by reference to the number of tokens returned.
 *
 * On out of memory, zero length string, zero length
 * separator, NULL is returned.
 *
 * Note that 'sep' is able to split a string using
 * a multi-character separator. For example
 * sdssplit("foo_-_bar","_-_"); will return two
 * elements "foo" and "bar".
 *
 * This version of the function is binary-safe but
 * requires length arguments. sdssplit() is just the
 * same function but for zero-terminated strings.
 */
 /* 使用sep字符切分字符串s,返回的是一个字符串数组
  * const char *s : 表示需要切分的字符串
  * ssize_t len : 字符串长度
  * const char *sep : 以该字符串或字符串来切分s
  * int seplen : sep字符或字符串的长度
  * int *count : 计数
  */
sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count) {
    int elements = 0, slots = 5; // elements：切的元素个数
    long start = 0, j; // start:从哪里开始
    sds *tokens; // 切分的字符串数组

    if (seplen < 1 || len < 0) return NULL; // 如果切分标志字符串或字符串长度小于0

    tokens = s_malloc(sizeof(sds)*slots); // 为数组分配内存
    if (tokens == NULL) return NULL;

    if (len == 0) { // 需要切分的长度为0
        *count = 0;
        return tokens;
    }
    for (j = 0; j < (len-(seplen-1)); j++) {
        /* make sure there is room for the next element and the final one */
        // 确保有空间能容纳下一个元素和最后一个元素
        if (slots < elements+2) { // 如果不能容纳就扩容(两倍)
            sds *newtokens;

            slots *= 2;
            newtokens = s_realloc(tokens,sizeof(sds)*slots);
            if (newtokens == NULL) goto cleanup;
            tokens = newtokens;
        }
        /* search the separator */
        // 搜索分割符
        // 如果分割符只是一个字符，直接字符比较就行了
        // 如果分割符是一个字符串，则需要使用内存比较
        if ((seplen == 1 && *(s+j) == sep[0]) || (memcmp(s+j,sep,seplen) == 0)) {
            tokens[elements] = sdsnewlen(s+start,j-start); // 将分割符前的，s当前指针后的字符串赋值给字符数组
            if (tokens[elements] == NULL) goto cleanup;
            // 相应的分割的元素个数+1,指针s的游标位置略过分割符，循环标志j也跳过分割符
            elements++; 
            start = j+seplen;
            j = j+seplen-1; /* skip the separator */
        }
    }
    /* Add the final element. We are sure there is room in the tokens array. */
    tokens[elements] = sdsnewlen(s+start,len-start); // 切分完后，需要将最后的元素放入字符数组
    if (tokens[elements] == NULL) goto cleanup;
    // *count记录元素总数
    elements++;
    *count = elements;
    return tokens;

// goto语句跳转的代码块，用来初始化
cleanup:
    {
        int i;
        for (i = 0; i < elements; i++) sdsfree(tokens[i]);
        s_free(tokens);
        *count = 0;
        return NULL;
    }
}

/* Free the result returned by sdssplitlen(), or do nothing if 'tokens' is NULL. */
// 用来释放sdssplitlen函数产生的字符串数组中指定的字符串元素内存
void sdsfreesplitres(sds *tokens, int count) {
    if (!tokens) return;
    while(count--)
        sdsfree(tokens[count]);
    s_free(tokens);
}

/* Append to the sds string "s" an escaped string representation where
 * all the non-printable characters (tested with isprint()) are turned into
 * escapes in the form "\n\r\a...." or "\x<hex-number>".
 *
 * After the call, the modified sds string is no longer valid and all the
 * references must be substituted with the new pointer returned by the call. */
 // 将转义字符一起连接打印(*p输入的是转义字符),用来解决sdscatprintf函数的局限
sds sdscatrepr(sds s, const char *p, size_t len) {
    s = sdscatlen(s,"\"",1); // 使用双引号将转义字符包围
    while(len--) {
        switch(*p) {
        case '\\': // 如果输入的是\和", 可以通过变量输入，不用转义
        case '"':
            s = sdscatprintf(s,"\\%c",*p);
            break;
        // 其他的转义字符就不能通过变量输入，就需要使用sdscatlen函数手动转义
        case '\n': s = sdscatlen(s,"\\n",2); break;
        case '\r': s = sdscatlen(s,"\\r",2); break;
        case '\t': s = sdscatlen(s,"\\t",2); break;
        case '\a': s = sdscatlen(s,"\\a",2); break;
        case '\b': s = sdscatlen(s,"\\b",2); break;
        default:// 如果输入的不是转义字符，就直接使用sdscatprintf函数打印
            if (isprint(*p)) 
                s = sdscatprintf(s,"%c",*p);
            else
                s = sdscatprintf(s,"\\x%02x",(unsigned char)*p);
            break;
        }
        p++;
    }
    return sdscatlen(s,"\"",1); // 用后引号包围
}

/* Helper function for sdssplitargs() that returns non zero if 'c'
 * is a valid hex digit. */
 // 判断是否是16进制
int is_hex_digit(char c) {
    return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') ||
           (c >= 'A' && c <= 'F');
}

/* Helper function for sdssplitargs() that converts a hex digit into an
 * integer from 0 to 15 */
 // 将16进制转换为10进制
int hex_digit_to_int(char c) {
    switch(c) {
    case '0': return 0;
    case '1': return 1;
    case '2': return 2;
    case '3': return 3;
    case '4': return 4;
    case '5': return 5;
    case '6': return 6;
    case '7': return 7;
    case '8': return 8;
    case '9': return 9;
    case 'a': case 'A': return 10;
    case 'b': case 'B': return 11;
    case 'c': case 'C': return 12;
    case 'd': case 'D': return 13;
    case 'e': case 'E': return 14;
    case 'f': case 'F': return 15;
    default: return 0;
    }
}

/* Split a line into arguments, where every argument can be in the
 * following programming-language REPL-alike form:
 *
 * foo bar "newline are supported\n" and "\xff\x00otherstuff"
 *
 * The number of arguments is stored into *argc, and an array
 * of sds is returned.
 *
 * The caller should free the resulting array of sds strings with
 * sdsfreesplitres().
 *
 * Note that sdscatrepr() is able to convert back a string into
 * a quoted string in the same format sdssplitargs() is able to parse.
 *
 * The function returns the allocated tokens on success, even when the
 * input string is empty, or NULL if the input contains unbalanced
 * quotes or closed quotes followed by non space characters
 * as in: "foo"bar or "foo'
 */
 // *line表示一个参数行，通过空格隔开;argc存储这个参数行每个参数的索引号
sds *sdssplitargs(const char *line, int *argc) {
    const char *p = line;
    char *current = NULL;
    char **vector = NULL; // 一个存放字符串的一个向量

    *argc = 0;// 第一个索引
    while(1) {
        /* skip blanks */
        while(*p && isspace(*p)) p++; // 如果当前指针游标指向的字符是空格，就跳过
        if (*p) {
            /* get a token */
            int inq=0;  /* set to 1 if we are in "quotes" */ // 如果在双引号中设为1
            int insq=0; /* set to 1 if we are in 'single quotes' */ // 如果p在单引号中设为1
            int done=0;

            if (current == NULL) current = sdsempty();
            while(!done) {
                if (inq) { // 如果p在双引号中,有16进制就转换为字符串
					// 如果p是转义字符，并且p的下一个是x,再下一个是16进制(\x19就是一个16进制)
                    if (*p == '\\' && *(p+1) == 'x' &&
                                             is_hex_digit(*(p+2)) &&
                                             is_hex_digit(*(p+3)))
                    {
                        unsigned char byte;
						// 将16进制转换为10进制再相加，得到一个字符
                        byte = (hex_digit_to_int(*(p+2))*16)+
                                hex_digit_to_int(*(p+3));
                        current = sdscatlen(current,(char*)&byte,1);//将这个字符连接到字符串后面
                        p += 3; // 指针游标向后偏移三位
                    } else if (*p == '\\' && *(p+1)) { // 如果是转义字符
                        char c;

                        p++;
                        // 将转义字符转义为普通字符
                        switch(*p) {
                        case 'n': c = '\n'; break;
                        case 'r': c = '\r'; break;
                        case 't': c = '\t'; break;
                        case 'b': c = '\b'; break;
                        case 'a': c = '\a'; break;
                        default: c = *p; break;
                        }
                        current = sdscatlen(current,&c,1); // 将转义字符连接到字符串后面
                    } else if (*p == '"') { // 如果再遇到双引号，就完成
                        /* closing quote must be followed by a space or
                         * nothing at all. */
                         // 如果指针的下一个内容不为NULL，并且其不是一个空格,那就报错
                        if (*(p+1) && !isspace(*(p+1))) goto err;
                        done=1;
                    } else if (!*p) { // 没有定义闭合双引号
                        /* unterminated quotes */
                        goto err;
                    } else { // 否则是正常字符
                        current = sdscatlen(current,p,1);
                    }
                } else if (insq) { // 如果是单引号，处理方法与处理双引号一样
                    if (*p == '\\' && *(p+1) == '\'') {
                        p++;
                        current = sdscatlen(current,"'",1);
                    } else if (*p == '\'') {
                        /* closing quote must be followed by a space or
                         * nothing at all. */
                        if (*(p+1) && !isspace(*(p+1))) goto err;
                        done=1;
                    } else if (!*p) {
                        /* unterminated quotes */
                        goto err;
                    } else {
                        current = sdscatlen(current,p,1);
                    }
                } else { // 如果没有双引号和单引号
                    switch(*p) {
                    case ' ': // 遇到转义符号就需要结束
                    case '\n':
                    case '\r':
                    case '\t':
                    case '\0':
                        done=1;
                        break;
                    case '"': // 如果遇到双引号
                        inq=1;
                        break;
                    case '\'': // 如果遇到单引号
                        insq=1;
                        break;
                    default: // 否则就是普通字符
                        current = sdscatlen(current,p,1);
                        break;
                    }
                }
                if (*p) p++; // 只要p不为空，就接着遍历
            }
            /* add the token to the vector */
            // 为向量分配内存
            vector = s_realloc(vector,((*argc)+1)*sizeof(char*));
            vector[*argc] = current; // 将元素放入向量中
            (*argc)++;
            current = NULL;
        } else {// 到达一行结尾，如果向量还没有存入元素，不能让向量返回NULL，也需要为向量分配内存空间
            /* Even on empty input string return something not NULL. */
            if (vector == NULL) vector = s_malloc(sizeof(void*));
            return vector;
        }
    }
// 如果遇到错误就释放内存，向量返回NULL
err:
    while((*argc)--)
        sdsfree(vector[*argc]);
    s_free(vector);
    if (current) sdsfree(current);
    *argc = 0;
    return NULL;
}

/* Modify the string substituting all the occurrences of the set of
 * characters specified in the 'from' string to the corresponding character
 * in the 'to' array.
 *
 * For instance: sdsmapchars(mystring, "ho", "01", 2)
 * will have the effect of turning the string "hello" into "0ell1".
 *
 * The function returns the sds string pointer, that is always the same
 * as the input pointer since no resize is needed. */
 // 将字符串s中的from字符串根据to字符串按顺序替换
sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen) {
    size_t j, i, l = sdslen(s);

    for (j = 0; j < l; j++) {
        for (i = 0; i < setlen; i++) {
            if (s[j] == from[i]) {
                s[j] = to[i];
                break;
            }
        }
    }
    return s;
}

/* Join an array of C strings using the specified separator (also a C string).
 * Returns the result as an sds string. */
 /* 将标准C字符串连接为sds字符串
  * **argv: ["aaa" , "bbb" , "ccc"]
  * *sep : "-"
  * argc: 3 
  * result : "aaa-bbb-ccc"
  */
sds sdsjoin(char **argv, int argc, char *sep) {
    sds join = sdsempty(); // 得到一个空的sds字符串
    int j;

    for (j = 0; j < argc; j++) { 
        join = sdscat(join, argv[j]); // 将字符串数组中的字符串连接到join字符串后面
        if (j != argc-1) join = sdscat(join,sep); // 每个数组中的字符串以分割符sep连接
    }
    return join;
}

/* Like sdsjoin, but joins an array of SDS strings. */
//将sds字符串数组中的sds字符串连接为一个字符串;与sdsjoin功能相似
sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen) {
    sds join = sdsempty();
    int j;

    for (j = 0; j < argc; j++) {
        join = sdscatsds(join, argv[j]);
        if (j != argc-1) join = sdscatlen(join,sep,seplen);
    }
    return join;
}

/* Wrappers to the allocators used by SDS. Note that SDS will actually
 * just use the macros defined into sdsalloc.h in order to avoid to pay
 * the overhead of function calls. Here we define these wrappers only for
 * the programs SDS is linked to, if they want to touch the SDS internals
 * even if they use a different allocator. */
 // 封装的sds分配内存函数(相当与继承自s_系列分配函数,而s_系列又是继承自z_系列，z_系列又是继承自系统分配函数)
void *sds_malloc(size_t size) { return s_malloc(size); }
void *sds_realloc(void *ptr, size_t size) { return s_realloc(ptr,size); }
void sds_free(void *ptr) { s_free(ptr); }

// 下面是用来测试的,也是上面函数的使用方法，如果不清楚上面函数怎么使用，可以参考一下
#if defined(SDS_TEST_MAIN)
#include <stdio.h>
#include "testhelp.h"
#include "limits.h"

#define UNUSED(x) (void)(x)
int sdsTest(void) {
    {
        sds x = sdsnew("foo"), y;

        test_cond("Create a string and obtain the length",
            sdslen(x) == 3 && memcmp(x,"foo\0",4) == 0)

        sdsfree(x);
        x = sdsnewlen("foo",2);
        test_cond("Create a string with specified length",
            sdslen(x) == 2 && memcmp(x,"fo\0",3) == 0)

        x = sdscat(x,"bar");
        test_cond("Strings concatenation",
            sdslen(x) == 5 && memcmp(x,"fobar\0",6) == 0);

        x = sdscpy(x,"a");
        test_cond("sdscpy() against an originally longer string",
            sdslen(x) == 1 && memcmp(x,"a\0",2) == 0)

        x = sdscpy(x,"xyzxxxxxxxxxxyyyyyyyyyykkkkkkkkkk");
        test_cond("sdscpy() against an originally shorter string",
            sdslen(x) == 33 &&
            memcmp(x,"xyzxxxxxxxxxxyyyyyyyyyykkkkkkkkkk\0",33) == 0)

        sdsfree(x);
        x = sdscatprintf(sdsempty(),"%d",123);
        test_cond("sdscatprintf() seems working in the base case",
            sdslen(x) == 3 && memcmp(x,"123\0",4) == 0)

        sdsfree(x);
        x = sdsnew("--");
        x = sdscatfmt(x, "Hello %s World %I,%I--", "Hi!", LLONG_MIN,LLONG_MAX);
        test_cond("sdscatfmt() seems working in the base case",
            sdslen(x) == 60 &&
            memcmp(x,"--Hello Hi! World -9223372036854775808,"
                     "9223372036854775807--",60) == 0)
        printf("[%s]\n",x);

        sdsfree(x);
        x = sdsnew("--");
        x = sdscatfmt(x, "%u,%U--", UINT_MAX, ULLONG_MAX);
        test_cond("sdscatfmt() seems working with unsigned numbers",
            sdslen(x) == 35 &&
            memcmp(x,"--4294967295,18446744073709551615--",35) == 0)

        sdsfree(x);
        x = sdsnew(" x ");
        sdstrim(x," x");
        test_cond("sdstrim() works when all chars match",
            sdslen(x) == 0)

        sdsfree(x);
        x = sdsnew(" x ");
        sdstrim(x," ");
        test_cond("sdstrim() works when a single char remains",
            sdslen(x) == 1 && x[0] == 'x')

        sdsfree(x);
        x = sdsnew("xxciaoyyy");
        sdstrim(x,"xy");
        test_cond("sdstrim() correctly trims characters",
            sdslen(x) == 4 && memcmp(x,"ciao\0",5) == 0)

        y = sdsdup(x);
        sdsrange(y,1,1);
        test_cond("sdsrange(...,1,1)",
            sdslen(y) == 1 && memcmp(y,"i\0",2) == 0)

        sdsfree(y);
        y = sdsdup(x);
        sdsrange(y,1,-1);
        test_cond("sdsrange(...,1,-1)",
            sdslen(y) == 3 && memcmp(y,"iao\0",4) == 0)

        sdsfree(y);
        y = sdsdup(x);
        sdsrange(y,-2,-1);
        test_cond("sdsrange(...,-2,-1)",
            sdslen(y) == 2 && memcmp(y,"ao\0",3) == 0)

        sdsfree(y);
        y = sdsdup(x);
        sdsrange(y,2,1);
        test_cond("sdsrange(...,2,1)",
            sdslen(y) == 0 && memcmp(y,"\0",1) == 0)

        sdsfree(y);
        y = sdsdup(x);
        sdsrange(y,1,100);
        test_cond("sdsrange(...,1,100)",
            sdslen(y) == 3 && memcmp(y,"iao\0",4) == 0)

        sdsfree(y);
        y = sdsdup(x);
        sdsrange(y,100,100);
        test_cond("sdsrange(...,100,100)",
            sdslen(y) == 0 && memcmp(y,"\0",1) == 0)

        sdsfree(y);
        sdsfree(x);
        x = sdsnew("foo");
        y = sdsnew("foa");
        test_cond("sdscmp(foo,foa)", sdscmp(x,y) > 0)

        sdsfree(y);
        sdsfree(x);
        x = sdsnew("bar");
        y = sdsnew("bar");
        test_cond("sdscmp(bar,bar)", sdscmp(x,y) == 0)

        sdsfree(y);
        sdsfree(x);
        x = sdsnew("aar");
        y = sdsnew("bar");
        test_cond("sdscmp(bar,bar)", sdscmp(x,y) < 0)

        sdsfree(y);
        sdsfree(x);
        x = sdsnewlen("\a\n\0foo\r",7);
        y = sdscatrepr(sdsempty(),x,sdslen(x));
        test_cond("sdscatrepr(...data...)",
            memcmp(y,"\"\\a\\n\\x00foo\\r\"",15) == 0)

        {
            unsigned int oldfree;
            char *p;
            int step = 10, j, i;

            sdsfree(x);
            sdsfree(y);
            x = sdsnew("0");
            test_cond("sdsnew() free/len buffers", sdslen(x) == 1 && sdsavail(x) == 0);

            /* Run the test a few times in order to hit the first two
             * SDS header types. */
            for (i = 0; i < 10; i++) {
                int oldlen = sdslen(x);
                x = sdsMakeRoomFor(x,step);
                int type = x[-1]&SDS_TYPE_MASK;

                test_cond("sdsMakeRoomFor() len", sdslen(x) == oldlen);
                if (type != SDS_TYPE_5) {
                    test_cond("sdsMakeRoomFor() free", sdsavail(x) >= step);
                    oldfree = sdsavail(x);
                }
                p = x+oldlen;
                for (j = 0; j < step; j++) {
                    p[j] = 'A'+j;
                }
                sdsIncrLen(x,step);
            }
            test_cond("sdsMakeRoomFor() content",
                memcmp("0ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJ",x,101) == 0);
            test_cond("sdsMakeRoomFor() final length",sdslen(x)==101);

            sdsfree(x);
        }
    }
    test_report()
    return 0;
}
#endif

#ifdef SDS_TEST_MAIN
int main(void) {
    return sdsTest();
}
#endif